Oxygen can be separated from oxygen-containing gases by mixed-conducting ceramic membranes operating at high temperatures wherein the membranes conduct both oxygen ions and electrons. Oxygen gas is produced on the permeate side of the membrane and can be recovered as a high-purity product. Alternatively, the permeated oxygen can be reacted directly with a hydrocarbon-containing gas, either catalytically or non-catalytically, to yield a hydrocarbon oxidation product. Various oxygen-containing gases, such as air, can be used and numerous alternative hydrocarbon oxidation products are possible depending on the operating conditions and catalyst if used.
There is a significant and growing commercial interest in the production of synthesis gas from natural gas and air using mixed-conducting ceramic membrane reactor systems. This technology is presently in the development stage and commercial applications are envisioned in future years as the technology matures. Mixed-conducting ceramic membrane reactor systems produce synthesis gas by the partial oxidation of methane to form the synthesis gas components CO, H2, CO2, and H2O. The process is carried out by introducing a methane-containing feed gas and an air feed gas into the membrane reactor system, contacting one surface of the membrane with methane, and contacting the other surface with air. Oxygen permeates through the membrane, methane reacts with permeated oxygen to form a methane/synthesis gas mixture, and methane is further converted into synthesis gas as the mixture travels through the reactor while reacting with additional permeated oxygen.
This process can be integrated favorably with upstream and downstream processes if the methane/synthesis gas stream is at a high pressure, typically 250-450 psig. In addition, process economics are most favorable if the air is at a low pressure, typically less than 50 psig. Therefore, the membranes in the membrane reactor system must be designed to withstand a significant pressure differential between the air side and the methane/synthesis gas side. To achieve high oxygen fluxes through the membrane, the active separating layer of the membrane should be thin, typically less than 200 microns. However, a freestanding membrane of this thickness could not withstand a typical pressure differential of 200-400 psid, and the thin separating layer therefore must be structurally supported in some fashion.
Various designs for ceramic oxygen-conducting membrane systems capable of withstanding high pressure differentials have been described in the art. For example, a tubular ceramic membrane can be subjected to high pressure methane on one side and low pressure air on the other side, but such a membrane must have a sufficiently thick wall to withstand the pressure differential; as a consequence, this membrane cannot achieve a high oxygen flux. To address this problem, composite tubular membranes have been developed which incorporate a thin, dense oxygen-permeating layer on a thicker porous support.
Flat plate membrane configurations have been described in the art wherein the active separating layer is supported by a porous layer or layers on the low pressure side of the membrane, which typically is the permeate side of the membrane. These membrane systems typically are designed to produce a pure oxygen product on the permeate side. If these membranes are used with low pressure air on the low pressure side of the membrane, the porous support layers on the low pressure side of the membrane introduce a gas phase diffusional resistance for transport of oxygen from the oxidant or air to the surface of the dense separating layer. Porous layers that are thick enough to provide support for the thin active separating layer introduce diffusional resistance to the transport of oxygen to the membrane surface, and this resistance will decrease the oxygen flux through the membrane. A need exists, therefore, for composite membrane designs that use a thin active membrane under a high pressure differential without unacceptably high gas phase diffusional resistance on the oxidant side of the membrane.
Porous materials have lower mechanical strength than dense materials. Membrane designs that use porous supports on the low pressure side of a membrane subject the porous support to a compressive stress. This stress may exceed the crush strength of the porous support layer if the differential pressure is high enough, causing the support layer to fail and the thin active layer to leak or fail. The strength of a porous layer is a function of the porosity of the layer material—a lower porosity material is generally stronger than a higher porosity material. Unfortunately, a stronger material with a lower porosity is less permeable than a weaker material with a higher porosity, and increasing the strength of a porous support layer thus increases the gas phase diffusional resistance of the layer. This tradeoff between strength and permeability in porous support materials makes it difficult to design composite membranes that can withstand high pressure differentials and the resulting high compressive stresses. Thus there is a need for membrane designs that avoid placing porous layers under high compressive stresses.
Oxygen transport through a dense oxygen-conducting ceramic membrane is thermally activated. This means that the oxygen flux through the membrane increases exponentially with temperature in the absence of any other mass transfer resistances. When a dense oxygen-conducting membrane is used in a membrane reactor system to conduct an exothermic reaction such as hydrocarbon oxidation, the thermally activated oxygen transport can lead to local hot spots on the membrane. A thin spot on the membrane will experience a higher oxygen flux relative to thicker surrounding regions on the membrane, and the membrane will heat up at this thin spot relative to its surroundings as the oxidation rate increases. This will increase the flux further, thereby further increasing the temperature at that spot. These local temperature gradients generate undesirable thermal stresses that are detrimental to the mechanical integrity of the membrane.
There is a need in the ceramic membrane reactor field for a membrane design capable of withstanding high pressure differentials while also preventing local hot spots from occurring. In particular, there is a need for a hydrocarbon partial oxidation reactor membrane design that will allow the use of a thin oxygen-permeable membrane layer operating under a large pressure differential without developing hot spots caused by localized high oxygen diffusion and high exothermic oxidation rates. This need is addressed by the present invention as described below and defined by the claims that follow.